Flared horn wave guide antenna



Nov. 11, 1952 VAN ATTA 2,617,937

FLARED HORN WAVE GUIDE ANTENNA Filed Sept. 14, 1945 INVENTOR LESTER C. VAN ATTA Ox/www- WQ/L ATTORNEY Patented Nov. 11, 1952 FLARED HORN WAVE GUIDE ANTENNA Lester C. Van Atta, Winchester, Mass., assignor, by mesne assignments, to the United States of America as represented by theSecretary of War Application September 14, 1945, Serial No. 616,368

5 Claims.

This invention relates to antennas for communication systems and is particularly directed to an antenna having means for shaping the primary radiation pattern of a beam of electromagnetic energy emitted by a radiating element, such asa horn-type feed.

Such a radiating element has wide application. It can be used to match a wave guide transmis- The gain, half power beam width and side lobes of the radiation pattern of an antenna havinga paraboloidal reflector depend upon the phase and amplitude distribution of the radiation pattern of the reflector feed. The latter may be referred to as the primary pattern of the feed. For a circular dish-like reflector, the problem'of feed is relatively simple. For a truncated paraboloidal reflector it is usually necessary to use a feed having a sharper beam in one plane than in the plane perpendicular to it.

One objective in designing a paraboloid antenna is to realize the maximum possible gain consistent with side lobe requirements. In order to get the highest gain, it is required to have a relatively uniform illumination across the aperture of the paraboloid and low spill-over at the edge. To achieve this, one can either shape the primary beam of a feed to fit a given reflector, or v shape the reflector such as to form a truncated paraboloid to fit a given feed, or both.

The gain of a paraboloid antenna with a given feed is a maximum if the amplitude at the edge is half the average amplitude of the illumination over the aperture of the paraboloid. For a,

given feed, this provides a simple criterion for determining the size of the paraboloid. In general, the gain factor does not drop sharply with the increase of the aperture of the dish. Thus,

a slightly bigger aperture can always be used to reduce the side lobes of the radiation pattern of the antenna. In most cases, however, the converse problem is encountered; namely, for a given paraboloid, how to shape the primary paty tern of a feed to achieve the best results.

satisfactory from the standpoint of size as an increase in aperture means a fourfold increase in length which makes the horn difiicult to support. Furthermore, because of spilling over the edge of the dish and severely tapered illumination, the gain of a paraboloid antenna is usually low. In order to design an antenna of a high gain factor, it is necessary to shape a primary pattern of some special type to provide more uniform illumination with little energy shooting outside the reflector.

The principal object of the invention is to provide a horn type radiating element with means for controlling the primary pattern of the radiating element.

Another object of the invention is to provide a radiating feed element for an antenna having means adapted to furnishan approximation to uniform intensity across the aperture of the element for feeding a cut or truncated paraboloid. This may be accomplished according to' this invention by controlling the phasing of the first and third harmonic amplitudes of the radiated energy by careful design of the dimensions of a box type horn so that the phase and relative amplitude of the first and third modes can be combined to form the desired result.

It is still another object of the invention to provide a novel horn-like radiating element adapted to produce a primary pattern of correct half power width and beam shape and which is more compact, smaller and lighter in construction than elements heretofore known.

Other objects, advantages and novel features of the invention will become apparent from the following description with reference to the accompanying drawing which is a perspective view of a box horn type radiating element according to one embodiment of the invention.

In the drawing, l0 designates the box horn radiating element according to this invention comprising a hollow pipe wave guide I I, a flared portion 12 and a box-like structure 13. Wave guide II is of rectangular cross-section, having parallel Walls ofwider dimension Wg, and is adapted to transmit energy of suitable modes-and. frequencies of operation from a source (not shown). Preferably the width Wg is made sufiicient to support only the TE1,0 mode of energy.

Flared portion 12 comprises upper and lower walls M and i5 and side walls 16, each having one end joined to the respective walls of wave guide I I at throat 18. The opposite ends of the walls define rectangular mouth I9. Flared portion I2 may be flared in either or both of the H and E-planes (parallel to the directions of the H and E-vectors respectively). For example,

- aperture 20.

'ilared portion I 2 is shown in the drawing as preferably being flared in the H-plane only, that is the upper and lower walls I4 and I5 are disposed at preferably equal angles of 4: with respect to the respective walls of wave guide II. flared portion I2 has a wider dimention We at mouth I9 than. at throat I8. Similarly, flared portion 12 may be flared in the E-plane wherein the dimensions between the side walls I6 are larger at mouth I9 than at throat [8.

Structure I3 is a rectangular box-like structure having one end open defining aperture 20;

the wide dimension Wb of which is larger than the width of mouth I 9 of flared portion .I 2. Preferably the narrow dimension n is equal to the corresponding dimension of flared portion I2 along the length of structure I3. l-Iowever, if desired, the narrow dimension may also beflared outwardly away from flared portion I2 for at least a portion of the length of structure I3 to assist-in correcting-the impedance mismatch at The wall of structure I3 opposite to aperture 20 and which may be termed the rear "wall 2| is provided with an opening disposed substantially centrally of the width of the wall and 'equaltothe dimensions of mouth I9. 'Flared -po'rtion12 is joined along the edges defining -mouth' I9 tothe edges of the opening in the rear Wall 2 I. Thus, flared portion IZJopens into struc- "ture -'I 3 permitting energy to betransmittedifrom waveguide I I, through flared portion I2 and box- 'like structure1l3 and radiated from aperture 20. The width We of aperture 20,1isisuiiiciently zlarger. than width W11 so. as to permit the exist- ;encerin structure IS-of. botht-the TE1,0 and TE modes of the-energy propagated in" wave guide II. .'Evenimodes vare eliminated'by thesymmetrical'designand arrangement of the three *parts. of =box1horn I 0.

.The wave front of energyacrossmouth I9 will be :substantially circular as :indicated 1 by .the

-'.-*brokeni'linefP, due to .therflaring of portion. .12.

They-length Lh andtheLflare. angle sof :flared :portionJZ are chosen so that the phase variation D across Waispreferablyas small" as possible,

rat;

andif it is assumed, forexample, that'the energy acrosswn be out of phase by Xfheingthewavelength at the desired frequency 'ot operation; then 'Eromthe geometry ortheifigure and calculation isi determined by h a 2L,

'in-radians.

.L'The'ratio of amplitudes of the two modes,

that is of theflrst and third modes depends on firom'the center line of box-horn It. With con- Thus5 and .these amplitudes are in, phasezat the rear of structure I3. Slight variations of the phase .-across W11 are not found to afiect the results appreciably.

.2 Now let "Wb be restricted to the range 3/2 Wb/ 5/ 2, so that only the first and third modes are propagated to the aperture 20 of the box without-rapid" attenuation. The two modes have diiTerent wavelengths in the box so that their relativephase at aperture 2!] depends upon the lengthLb of structure I3. If A1 first mode box wavelength, and A3 third mode box wave- Calculation of the relative amplitude -F (0) radiated at an angle 6 from the horn axis A is made under the following assumptions. ((1.) Each portion of the aperture field radiates in a cos (0/2) distribution, arising from the directional derivative of the field. (b) There is no radiation other than from'the aperture field. (c) There are no multiple reflections withinthe box which contribute to the-aperture field.

Thus, for constant total power and given aperture dimensions,

if; desired,i.be=. adjusted. by: providing. outstanding :fiangesa along the wide. edges. oi'aperture. 20 and preferably: perpendicular to 1 the wide' walls of "structure; .I3 (in :the ;E-plane). Thus .E-plane lflangestmayz-zextend outwardly preferably for; a

distancepithetorder. of a'quarter wavelength.

For; a given..H-.plane' width: of "the radiation pattern, a flared boxhorn as described'in accordance with. this .invention :is considerably smaller in length and aperture .than. ausual type radiatinghorn. -The'iollowing tableillustrates the size .advantage of aibox. horn according to this invention over. ordinary radiating horns:

.;For a primary .beam pattern witha .full width of 60 between one tenth power points:

"It will be understood that the box horn I0 as described is adapted-to produce the desired radia- "tion pattern forilluminating a cut parabolic refiector and includes the desired advantages of more uniform intensity of the pattern, higher gain, smaller dimensions and more compact and lighter construction than horn type radiating elements heretofore used. 5

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention.

What is claimed is:

1. An antenna radiating element for radiating electromagnetic energy of a predetermined frequency, said element comprising a rectangular first wave guide having a uniform given width and a uniform given depth, the width and depth of said first wave guide being sufficient for said wave guide to support the TE1,0 mode of said energy, but insufficient for said first wave guide to support higher modes thereof; a second wave guide having a rectangular cross-section, the width and depth respectively of said second wave guide at one end thereof being equal to said given width and depth respectively, the width of said second wave guide at the other end thereof being greater than said given width and the depth of said second wave guide at said other end thereof being at least as great as said given depth, whereby said second wave guide forms a flared horn having its throat at said one end thereof and its mouth at said other end thereof; a third wave guide having a rectangular cross-section, the width of said third wave guide at one end thereof being greater than the width of said mouth of said second wave guide and the depth of saidthird wave guide at said one end thereof, being at least as great as said given depth, the width and depth of said third wave guide at the other end thereof being at least as great as at said one end thereof, said third wave guide having an end member across said one end thereof, said end member having an opening therein, the width and depth respectively of said opening being equal to the width and depth respectively of said 5 mouth of said second wave guide; one end of said first wave guide being connected to said second wave guide at the throat thereof, said second wave guide being connected to said third wave guide with said mouth in coincidence with said opening.

2. A radiating element as claimed in claim 1, wherein said opening is disposed centrally within said end member.

3. A radiating element as claimed in claim 1, wherein the width of said one end of said third wave guide is sufficient to support the TE3,0 mode of said energy, but insufficient to support higher modes.

4. A radiation element as claimed in claim 1, wherein the width of said one end of said third wave guide is sufiicient to support the TE3,0 mode of said energy, but insufficient to support higher modes thereof, and the ratio of the width of said one end of said third wave guide to the width of said mouth is substantially equal to the value that satisfies the equation:

where (As/A1) is equal to a predetermined desired value of the ratio of the amplitude of the energy in the TE3,0 mode to the amplitude of said TE1,0 mode in said structure and where is the ratio of the width of said mouth to the width of said one end of said third wave guide.

5. An antenna radiating element as recited in claim 4 where the length Lb of said third wave guide is computed from the formula where x is the wavelength of the desired frequency of operation and Wb is the width of said one end of said third wave guide, whereby the main lobe radiated is sharpened.

LESTER C. VAN ATTA.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Dallenbach: A. P. C. publication, S. N. 353,755, May 25, 1943. 

